Silicon is the main raw material of semiconductor industry, and it is estimated that the use of it will increase considerably in the production of electricity based on the solar radiation energy, provided that the production costs of silicon are substantially reduced from the cost level of present methods.
Silicon is industrially produced by reducing arenaceous quartz by carbon at a high temperature (DE patent application No. 30 232 97.4). The metallurgic silicon thus obtained is further purified by a treatment with hydrogen chloride gas, whereby various compounds of silicon, hydrogen and chloride arise, as for instance trichlorosilane and under certain circumstances also silane. The desired compounds are separated by distillation at a high temperature, e.g. the trichlorosilane is dissolved by hydrogen into silicon and hydrogen chloride. The disadvantage of this method is a high energy consumption and a low yield.
U.S. Pat. No. 2,172,969 discloses a method of reducing sodium silicon fluoride by sodium at a temperature of appr. 500.degree. C., the result of the reaction being an amorphous powder containing silicon and sodium fluoride. By treating the product thus obtained with sulphuric acid the silicon is separated, whereby the sodium fluoride reacts, constituting hydrogen fluoride and a solution of sodium sulphate. The purity of the silicon by this method is 96-97% and the yield approximately 87% of the theoretical yield.
A method of preparing silicon by reducing silicon tetrachloride or fluoride by sodium is also known. The basic reaction SiF.sub.4 +4Na.fwdarw.Si+4NaF has indeed been known for a very long time (Hempel et al. Zanorg. Chem. 23, 1900, 32/42). In the patented methods, the raw materials are purified by conducting them in the vapour phase through finely pulverized quartz and finely pulverized silicon (U.S. Pat. No. 3,041,145) or the silicon and the sodium fluoride are separated from each other by liquid phase separation by maintaining a temperature at the reactor bottom, which is above the fusion point of silicon (U.S. Pat. No. 4,442,082).
All the cited methods are characterized by that the reactions take place in the vapour phase and at a high temperature (probably more than 600.degree. C.), which e.g. means correspondingly high material costs, and the process is difficult to carry out as continuous action.